From practical applications, a multiplicity of security and/or value documents are known in the art, which comprise security features with luminescent, in particular fluorescent substances. Luminescent substances are such substances, which fluoresce or phosphoresce upon excitation with light having sufficient energy, for instance UV. These are energetic transition processes on a molecular or atomic level, the transition dipole moment of which is nonzero (fluorescence) or zero (phosphorescence). The wavelengths or energies of the fluorescence or of the phosphorescence are specific for the respective substances, since they correspond to the difference of the energy levels of the two states, between which a relaxation from the excited state takes place, and are in most cases in the visible range. The fluorescence typically has a decay time von 10 ns and less, since it is a dipole allowed transition (nonzero transition dipole moment), whereas the phosphorescence typically has decay times in the range from 1,000 μs up to several hours, since these are dipole forbidden transitions (zero transition dipole moment). Forbidden transitions have a comparably small transition probability, which leads to comparably slow transitions. The physical background of this behavior is for instance described in more detail in the document P. W. Atkins, Physikalische Chemie, 2nd edition, VCH, Weinheim, N.Y., Bale, Cambridge, Tokyo, 1996, pages 563 ff.
In particular, security features with fluorescent substances have the advantage that with simplest means a verification is possible, and that with a very economic production. When such a security feature is for instance held under a UV light source, it will light up and can directly be observed.
Security features with fluorescent substances are usually produced by means of fluorescent paints or inks, for instance by printing. Fluorescent paints or inks are widely used and can easily be procured. Therefore, it is easy for unauthorized persons, too, to procure a suitable fluorescent paint or ink and to make therewith forged security and/or value documents with a fluorescent security feature.
From other technological sectors, in particular the quantum well structures for laser diodes, so-called group II semiconductor contacts are known in the art. Reference is made for instance to the documents J. Am. Chem. Soc. 125:11466ff (2003), J. Appl. Phys. 87:1304ff (2000), Phys. Rev. B 36:3199ff (1987) and J. Am. Chem. Soc. 125:7100ff (2003). From the document U.S. Pat. No. 5,841,151, various group II semiconductor contacts based on InAsxPy and InpGaqAsxPy are known, and the two mentioned materials are directly contacted with each other and x and y on the one hand and p and q on the other hand always add up to 1. In this document, effects on the wave functions of holes and electrons are also described, which occur upon the application of a potential to the contact. Further similar contacts from two different group III/V semiconductors are for instance known from the document U.S. Pat. No. 6,734,464. From the document L. S. Braginsky et al. “Kinetics of exciton photoluminescence in type-II semiconductor lattices”, 2006, decay times of excitons for the system GaAs/AlAs (undoped) and the measurement thereof are known in the art. A detailed background representation of the band structures and wave functions in type II contacts is given further below.
It would be desirable to provide a security and/or value document with a luminescent security feature, which with continued simple production of the security and/or value document offers a higher security against forgery and an improved detectability of forgeries.